1. Field of the Invention
The present invention relates to a steam turbine blade made of Ti-base alloy, a method of manufacturing the same, a steam turbine power generating plant using the same and a low pressure steam turbine.
2. Description of the Related Art
Conventionally, in a low pressure final stage of a steam turbine, there have been developed 12Cr steel for a blade having 33.5 inch length, Ti-6Al-4V for a blade having 40 inch length, and high strength 12Cr steel for a blade having 43 inch length which is the longest in the world as a machine corresponding to 50 Hz, however, a demand for improving an efficiency and compactifying the plant in accordance that the final blade stage is made long is increased more and more, so that it is required to further lengthen the blade. In order to achieve the requirement, a titanium alloy having a light weight and a high strength is indispensable in place of Ti-6Al-4V which has been practically used.
A titanium alloy in class of tensile strength 95 kg/mm.sup.2 can sufficiently correspond to an increase of a centrifugal force caused by the blade having the increased length till the blade having 40 inch, however, in the blade having a length equal to or more than 45 inch, a titanium alloy in class of tensile strength 110 kg/mm.sup.2 is required. As the titanium alloy having a tensile strength equal to or more than 110 kg/mm.sup.2, there is a .beta. type titanium alloy having an age hardening property, however, since the .beta. type titanium alloy has a disadvantage, that is, a toughness is low, there is a problem in manufacturing a whole of the blade by this alloy. On the contrary, in an .alpha.+.beta. type titanium alloy having a high toughness, a cooling speed for a solid solution treatment largely affects the strength in accordance that a dovetail of the blade becomes thick, so that the strength which can be obtained in a small steel lump can not be frequently realized in a large-sized product. Accordingly, it has been hard to securely obtain a titanium alloy in class of 110 kg/mm.sup.2.
Further, in Japanese Patent Unexamined Publication No. 1-202389, there is described that a solid solution treatment is executed at a temperature equal to or less than 10 to 60.degree. C. corresponding to a point of .beta. transformation with respect to a condition for a heat treatment of Ti-6Al-6V-2Sn corresponding to an .alpha.+.beta. type high strength Ti alloy, that is, at 867 to 917.degree. C. and an age treatment is thereafter executed at 500 to 650.degree. C., however, in accordance with this treatment, there has been a problem that the strength can be obtained in a thin blade profile portion, but the strength can not be secured in a thick dovetail portion in which a cooling speed is low.
Further, in Japanese Patent Unexamined Publication No. 7-150316, there is described a turbine blade made of Ti-base alloy containing 3 to 5% of Al, 2.1 to 3.7% of V, 0.85 to 3.15% of Mo and 0.85 to 3.15% of Fe as a material for the turbine blade, however, there is not indicated an age treatment.